An interior permanent magnet (IPM) motor is primarily used in green vehicles, such as, an electric vehicle and a hybrid electric vehicle, because the IPM motor may provide high efficiency, high torque, and high output density. The IPM motor is classified into a concentrated winding type IPM motor or a distributed winding type IPM motor.
Whenever the IPM motor and a general motor rotate one cycle of an electrical angle, their torque periodically varies according to a predetermined frequency. The predetermined frequency may be determined based on combination of the number of phases, poles, and slots of the motor. A ratio of a torque variation difference and a torque average is referred to as torque ripple, and is generally determined in a percentage. For example, torque ripple of the IPM motor that is driven by 3-phase power and that is made with a pole/slot number ratio of 1/6 may occur at frequency components corresponding to 3, 6, 9, . . . times the pole number (N). Frequency components corresponding to 3 and 6 times the pole number (N) among the frequency components, for example, 24th-order and 48th-order frequency components of the IPM motor made with 3 phases, 8 poles, and 48 slots, generally cause audible sound.
An IPM according to the related art causes a noise associated with frequencies corresponding to 6 times the number (N) of poles of the IPM motor when revolutions per minute (rpm) is over a middle/high speed (e.g., 4000 rpm), and it is known that the noise gets on a driver's nerves. Since the noise and/or the torque ripple noise may aggravate noise, vibration, and harshness (NVH) of the green vehicles, such as the electric vehicle and the hybrid vehicle that use a motor, the noise and/or the torque ripple are/is a considered performance factor when the motor is designed.
However, design for reducing the torque ripple may worsen total harmonic distortion (THD) that influences controllability and iron loss, and increase a maximum current for driving.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure, and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.